1. Field of the Invention
This invention relates to an electrolysis system including an electrolyzer containing a plurality of bipolar electrolytic cells particularly suitable for the production of metal chlorates, particularly alkali metal chlorates. It relates, more specifically, to an electrolysis system including an improved electrolytic cell and apparatus containing multiple unit cells. The present invention also relates to an improved reactor for use in an electrolysis system.
2. Description of the Prior Art
Known electrolytic cells for the production of metal chlorate have certain disadvantages. Monopolar cells inherently have many power connections and electrolyte branches, gas phase above electrolyte level with electrode connectors extending through the gaseous zone with resulting danger of gas explosions and fires as well as high voltage drop; furthermore, many units are required in commercial production and large building space is occupied. Bipolar cells are designed for more compactness, but have other disadvantages and problems such as, for example, current leakage, channelling of electrolyte and gaseous products, as well as construction and assembly difficulties. Generally both types of cells employed graphite anodes until recent years when many of the commercial plants changed from graphite to dimensionally stable anodes of noble metal-coated titanium material. Almost all of the new installations favour the new anodes.
The benefits of metal electrodes in the manufacture of products, for example, chlorine-alkali, chlorate, perchlorates, etc., have been indicated in many publications; e.g. Canadian Patent No. 771,140 issued Nov. 7, 1967 to S.I. Burghardt relates to the advantages of metal electrodes; another Canadian Patent No. 631,022 issued Nov. 14, 1961 to R. G. Cottam and M. G. Derlez relates to anode improvements of said type.
One problem in employing metal electrodes in both monopolar and bipolar electrolytic cells is primarily that, because of the cost of the electrodes, electrode thicknesses are normally minimized. Also, in order to minimize power consumption, the cell gap is frequently quite small; e.g., a metal electrode suitable for chloride electrolysis may be relatively thin sheeting of titanium, for example, 0.5 to 3 mm, which is surface coated on each face to provide that best anode and in some cases cathode surfaces respectively. This coating may be from only a few to a few hundred microns in thickness. The optimum electrolytic cell gap for electrodes of this type and for the manufacture of, e.g., chlorate, will depend upon many factors, such as, for example, the surface coating of electrodes, the current density, the electrode height, the gas-to-liquor ratio, the electrolyte composition and the temperature. For conventional cell conditions in which, for example, the electrode height is 200 to 1500 mm, the current densities are from 1 to 4 KA per square meter and the gas/liquor ratio is approximately 1:1, and the optimum cell gap is probably between 1 and 10 mm.
Although certain inherent advantages accrue in the use of metal electrodes instead of graphite electrodes, monopolar and bipolar electrolytic cells are not generally designed for use with metal electrodes. Advantages of the use of metal electrodes including the following:
i. compact cells, because of the use of thin electrodes and a high current density; PA0 ii. lower power consumption, because of better surface properties, i.e., lower over-voltages, lower resistance in electrode material as well as in the electrolyte due to higher operating temperatures, and smaller average electrode gaps resulting in less voltage drop; PA0 iii. high operating temperature, thereby minimizing the requirement for heat exchangers in the system to provide temperature control; PA0 iv. provide for vaporization of water, in order to increase electrolyte and product strength; PA0 v. clean electrolyte, since metal electrodes do not normally show any significant mechanical erosion and subsequent charging of matter which is suspended in the electrolyte; and PA0 vi. less foam problems of the electrolyte, since the metal electrodes do not normally add ingredients to the electrolyte which would result in a foam problem which may be the case when employing, e.g., impregnated graphite electrodes.
The art of electrolytical manufacture of chlorates has developed significantly in recent years mainly because of the established excellent performance of the titanium surface-coated anodes referred to above. The electrolytic systems utilizing these anodes, however, generally are mere modifications of conventional apparatus and in most cases, comprise monopolar cell units suspending the anodes from the cell tank cover between steel cathode sheets welded to the cell tank. Thus, in recent years, there have been new developments both in the design of electrolytic cells and in the design of the electrodes disposed therein. Both monopolar and bipolar electrode types of system have been developed, some of which are used in commercial production
One of those systems is described in Canadian Patent No. 914,610 issued Nov. 14, 1972 to G. O. Westerlund, in the following terms: A novel electrolysis apparatus includes at least two modular monopolar electrolytic cells. Each such modular monopolar electrolytic cell includes an open-ended main chamber having inlet means for the flow of electrolyte to, and between, adjacent, parallel, alternately spaced anodes and cathodes, and outlet means constructed and arranged to withdraw electrolyte along with gaseous products of electrolysis entrained and/or occluded therein from the chamber, the main chamber being electrically isolated from the anodes and cathodes. An anode end plate is disposed at, and seals, one open end thereof, the anode end plate being provided with a plurality of spaced-apart anodes projecting from one face thereof into the main chamber. A cathode end plate is disposed at, and seals, the other open end, the cathode end plate being provided with a plurality of spaced-apart cathodes projecting from one face thereof into the main chamber in staggered alternate relationship to the anode also projecting into the main chamber. A common intermediate cathode-anode holding and current transmitting plate is disposed at, and seals, the adjacent open ends of two adjacent such cells. It is provided with a plurality of spaced-apart cathodes projecting from one face thereof into the main chamber in staggered alternate relationship to the anodes also projecting into the main chamber, and a plurality of spaced-apart anodes projecting from the other face thereof into the main chamber in staggered alternate relationship to the cathodes also projecting into the main chamber. The anodes and cathodes occupy less than the entire cross-sectional area of the main chamber, thereby to provide at least one non-electrolysis zone within each such cell. This enables internal liquor circulation resulting from gases evolved on the electrode surfaces to interchange electrolyte between the electrodes and to provide substantially homogeneous conditions in the cell chamber.
Although this design has a proven efficient performance, the construction is not one which can readily be carried out in the field. This is because the modular cell assembly comprises a plurality of electrode plates which must be carefully fitted when assembling the multiunit cell in order to avoid electrical short circuiting between adjacent cell modules. Cells designed for operation under low voltage conditions by having close spacing between electrodes are thus not readily maintained or constructed in the field. This disadvantage also applies to most other high efficiency electrolytic cells.
The above-identified Canadian Patent No. 914,610 also provides novel metal electrode contructions for electrolytic cells. The combined electrolyzer reactor employs an electrode arrangement where all anodes are welded to one side of a first carrier plate. A second carrier plate has matching cathode steel plates. In the electrolyzer the cathodes of the second carrier plate are fitted between the anodes of the first carrier plate. This requires hours of fitting for each cell in order to avoid the presence of any electrical short circuits. Capital cost would be high due to the tight tolerance limits required for satisfactory operating.
Recently significant progress was made in advancing the technology by the feature of module electrodes (H. B. Westerlund, U.S. Patent Appln. Ser. No. 618,078 filed September 30, 1975, now U.S. Pat. 3,994,798. Such modules may be described as comprising a plurality of modular bipolar electrode assemblies, each comprising: (1) a plate-like metallic anode formed of anode material; (2) a plate-like metallic cathode formed of cathode material; (3) a generally V-shaped in cross-section median electrode plate formed of titanium or a titanium alloy, interposed between, and connected to, each of the plate-like metallic anode and the plate-like metallic cathode, the median electrode extending below the bottom edge of the plate-like metallic anode and the plate-like metallic cathode; and (4) a plurality of electrically insulating spacer elements projecting outwardly from both side faces of at least the plate-like metallic cathode; and further including at least two median electrodes each interposed between, and connected to, a plate-like metallic anode and a plate-like metallic cathode, with the anodes and cathodes interleaved and spaced apart by the electrically non-conducting spacers, and with adjacent V-shaped median electrode plates in electrical connection with each other and adapted to provide current flow transversely of the assembly, which are disposed in a framework including a plurality of transversely extending titanium support plates within which the upwardly extending slot is accommodated, thereby to cooperate with the electrically connected median electrodes and adapted to provide current flow transversely of the assemblies.
Specifically referred to the chlorate manufacture, it is well known that alkali metal chlorates may be prepared by electrolysis of an aqueous solution of an alkali metal chloride. This process has been fully described in Canadian Patent No. 741,778 issued Aug. 30, 1966 to G. O. Westerlund.
This simplified reaction in the aforesaid electrolysis may be summarized as: EQU MtCl + 3H.sub.2 O + 6 Faradays -- MtClO.sub.3 + 3H.sub.2
(wherein Mt is a metal)
The main reactions in the electrolytic preparation of the metal chlorate from the metal chloride may be represented as follows: